[v0.6]: Fix GCP signed data gateway receipt URL, let deployer fail with non zero status code

[Siegrift]

Applies #1022 to the 0.6 release.

[Siegrift]

I've tested this successfully. Instructions for @kolenic-martin :

1. Check out this branch, use yarn build in the project root (needed to properly genearate config.json)
2. Go to examples, choose signed data example + GCP (network does not matter)
3. Run yarn create-airnode-config and yarn create-airnode-secrets
4. Download the GCP service account key: https://docs.api3.org/airnode/v0.6/grp-providers/docker/deployer-image.html#gcp
5. Edit the deploy-airnode and remove-airnode scripts to use the api3/airnode-deployer-dev:a39528e8282f3e65614748101d333168bc1ead28 image (built by the CI)
6. Run deploy-airnode
7. Make a successful signed data request
8. Run remove-airnode

Also, please check whether the signed data documentation whether it's accurate: https://docs.api3.org/airnode/v0.6/grp-providers/guides/build-an-airnode/http-gateways.html#http-signed-data-gateway (it looks fine to me, but I only tested using airnode-examples).

[amarthadan]

LGTM but leaving testing for @kolenic-martin